When drilling in an earth formation, it is previously known to measure a number of parameters or quantities “downhole” in the borehole, e.g. oil wells, by means of various sensors located in the bottom-hole assembly. The bottom-hole assembly is the unit which includes the drill bit and is connected to the bottom end of the drill string, and is thus positioned at the bottom of the bore hole. Said sensors are suitably located at a short distance behind the drill bit. The data generated from the sensors can be stored in a memory provided in the bottom-hole assembly the for later retrieval when the drill string is drawn out of the borehole, or can be encoded and transmitted to the surface via some kind of transmission system. For an operator, it is advantageous to receive said data at the surface during the drilling operation instead of waiting for the drill string to be drawn out of the borehole.
Further, the bottom-hole assembly can comprise a control unit controlling the drill bit, and other electronic or mechanical equipment. A bottom-hole assembly can be provided with different types of drill bits and associated equipment, e.g. a percussion drill bit and its percussion mechanism, commonly called the “hammer”, located directly behind the drill bit. The drill string transmits necessary feed force and rotation to the percussion mechanism and the drill bit, and also compressed fluid for the percussion mechanism, for example compressed air or liquid. The percussion mechanism can include a piston which is adapted to directly strike an impact surface of the drill bit. Since the percussion mechanism follows the drill bit down into the bore hole, the drilling method using this kind of bottom-hole assembly is called “down-the-hole” drilling.
In top-hammer drilling the percussion mechanism is instead situated on the drill rig, i.e. outside the bore hole.
A bottom-hole assembly can also comprise a rotary drill bit which is provided with rotating cutting elements.
There are several known methods for transmitting data from the sensors situated in the bottom-hole assembly to the surface. A common method for data transmission from the bottom-hole assembly, is mud pulse telemetry. Mud pulse telemetry can be divided into three categories: continuous wave telemetry, positive pulse telemetry, and negative pulse telemetry. In continuous wave telemetry, data from the downhole sensors is transmitted by a sinusoid type wave through the drilling mud (slurry) within the drilling pipe. Data is contained in the phase variation of this wave, and not in the amplitude.
In positive pulse telemetry, data from the downhole sensors are transmitted by briefly interfering with the mud flow within the drill pipe to produce an increase in pressure which can be detected at the surface.
Negative pulse telemetry, is generally the same as positive pulse telemetry, but a pressure decrease is used for the transmission of encoded data instead of a pressure increase. Whichever method is used, the generated waves are detected at the surface by surface mud pressure transducers. However, the mud pulse telemetry exhibits considerable data rate limitations and requires adequate mud.
Another method for data transmission from the bottom-hole assembly is electronic pulse telemetry. By voltage differences in the drill string, a pattern of low frequency waves is produced along the drill string. Data is modulated into these waves through phase alterations, similar to the continuous wave mud pulse telemetry, and the waves are detected at the surface. However, electronic pulse telemetry falls short when drilling exceptionally deep boreholes, when the signal can lose strength rapidly in some earth formations, and become undetectable at only a few thousand feet of depth.
According to yet another method for data transmission from the bottom-hole assembly, a system where electrical wires are built into every pipe of the drill string is used. The electrical wires carry electrical signals directly to the surface. Between the pipes, the wires are inductively connected to each other. This system promises greater data transmission rates in relation to the above-mentioned systems, both from the bottom-hole assembly to the surface, and from the surface to the bottom-hole assembly. However, this system is expensive, as the special drill pipes used are more expensive to produce in relation to conventional drill pipes. Additionally, this system is not entirely reliable. If a single pipe or a single connection between two pipes fails, the entire system fails.
GB 2 236 782 discloses an acoustic telemetry system, where an apparatus for acoustic telemetry along the drill string is provided. The apparatus includes a sensor adapted to generate an electrical signal representing a measured quantity, means for converting the electrical signal into a binary digital form, and a plurality of hammers arranged to be actuated successively to transmit successive binary digits by impacting with the drill string. Each hammer is adapted to deliver an impact to the drill string in one of two opposite directions, an impact in one direction representing the digit one and an impact in the opposite direction representing the digit zero.
WO 99/19751 discloses a telemetry system where stress and/or motion in a drill string is modulated for transmitting data uphole and downhole along the drill string located in a borehole, for example by varying the rate of the rotation of the drill string.
The drawback of most of the above-mentioned systems and methods for data transmission from the bottom-hole assembly to the surface is that the drilling must be interrupted during data transmission, or at least be interrupted for enabling a data transmission at an acceptable quality level. These interruptions are time consuming and result in increased costs for the drilling activity. Further drawbacks are limitations in data transmission rates and poor quality of the data transmission.